Well the default gateway that you would specify in the Scope Options of DHCP would be the IP of the Server. As for the default gateway in your tcp/ip properties on the server, that would normally be the default gateway provided to you by your ISP, but you're not wanting to connect to the internet, so I'm not sure, I'm pretty much a novice myself. Maybe just the loopback IP 127.0.0.1 or maybe not, that may cause problems so I would wait for further advice on that. If you do want to set it to the default gateway provided to you by your ISP for the other computers to have Internet access, then I believe that you will also need to add the IP addresses of your ISP's DNS servers as Forwarders in DNS on the 2000 Server.
As far as the Network Path Not Found error message, please let me know if you figure it out. I was helping a friend set up a small test Windows 2000 environment at his home, and at one point there was a 2000 Pro client that logged onto the 2000 Server no problem, then rebooted that machine into XP (dual boot) and tried logging in with XP, and couldn't join the domain (using administrator acct and computer acct was set up etc.) Then, without changing anything the XP client was able to log in, and the 2000 Pro client was getting the Network Path Not Found message... even though the two machines could ping each other no problem. Even though the XP and 2000 Pro client were the same machine, they had two different IP's and 2 separate computer accounts... very strange.